1. Field of the Invention
The present invention relates to color processing executed when a color conversion lookup table is generated.
2. Description of the Related Art
In order to control a printing apparatus to output desired colors, a color conversion lookup table (LUT) is used. The color conversion LUT includes an LUT used in calibration for maintaining a printing apparatus in a constant state, and that used in color matching, which is represented by International Color Consortium (ICC) profiles. Upon generating these color conversion LUTs, for example, a large number of color patches are output by the printing apparatus, and colors of the color patches are measured to obtain colorimetric values. Then, a color conversion LUT is generated by associating data of the color patches as device independent values input to the printing apparatus with the colorimetric values as device dependent values.
A general spectrophotometer measures a spectral reflectance which does not depend on a light source by dividing a spectral distribution obtained by irradiating an object to be measured with measurement light by that obtained by irradiating a perfect transmitting diffuser with identical measurement light. Then, using the spectral reflectance, a spectrum of a viewing light source, and a color matching function, the spectrophotometer outputs tristimulus values of a color of the object to be measured under a certain viewing light source as a colorimetric value.
On the other hand, most media (printing paper, developing paper, etc.) used in printing contain a fluorescence whitener which absorbs ultraviolet light (UV) and emits fluorescence in a visible range, so as to enhance a degree of white. Reflected light from a medium containing the fluorescence whitener includes reflected light components and fluorescence components from the medium, and a general spectrophotometer cannot accurately measure an object color of the medium containing the fluorescence whitener. This is because the reflected light components can be measured independently of the types of light sources, but the intensity and distribution of the fluorescence components change depending on the spectral distribution of a light source. For this reason, when measurement light used at the time of colorimetry is different from an actual viewing light source, luminescence by the fluorescence whitener is different, and an apparent spectral reflectance suffers a deviation.
In order to solve the aforementioned problem, the invention of Japanese Patent Laid-Open No. 10-176953 calculates an object color under a certain viewing light source by measuring the object color using measurement light in which UV is removed and that including UV. However, a whitening effect by a fluorescence whitener depends on temperatures, and colorimetric values of an image formed on a medium containing the fluorescence whitener depend on temperatures. The invention of Japanese Patent Laid-Open No. 10-176953 does not consider any temperature dependence of the whitening effect of the fluorescence whitener. In other words, in order to accurately generate a color conversion LUT for a medium containing a fluorescence whitener, an object color (to be referred to as paper white hereinafter) of a medium has to be measured in consideration of the influence of a fluorescence whitener contained in a medium, that is, the temperature dependence of the whitening effect.